Electrolyte for the deposition of



Patented Aug. 7, 1934- UNITED STATES PATENTZ- OFFICE ELECTROLYTE FOR THE DEPOSITION OF No Drawing. Application July 30, 1932,

Serial No. 627,126

4 Claims.

My invention relates to an electrolyte for the deposition of copper and copper alloys. Copper can be deposited very successfully from the acid copper sulphate bath, but its use is limited, be cause many of the common metals and alloys, which it would be desirable to plate will cause deposition by immersion when placed in the bath, or are attacked by the acid in the bath. Consequently, the cyanide baths have been used extensively for the deposition of copper and copper alloys, especially on iron and steel.. Although dense and adherent deposits can be obtained from the cyanide baths, they .are far from de- 4. Specific resistivity of cyanide solutions is high; r

5. Cyanide baths decompose slowly with formation of carbonates, necessitating additions of cyanide from time to time.

The function of the cyanide. ion in the cyanide baths for alloy deposition is to form complex ions with the ions of the metals to be deposited, in order to reduce the efiective metal ion concentrations, so that the single electrode potentials of the various metals will be brought sufliciently close to one another to permit an alloy to be deposited. When pure copper is to be deposited, the effective copper ion concentration is reduced far enough so that deposition by immersion will not occur. An object of my invention is, therefore, to provide a method involving the use of some other complex copper ion.

The full objects and advantages of my invention will appear in connection with the detailed description thereof, and the novel features of my inventive idea will be particularly. pointed out in the claims.

Thiosulphate ion appears to form a complex ion with cuprous ion as shown by the following facts:

1. Cuprous chloride has a much greater solubility in a 40% solution of NazSzOa.5Hz0 than in pure water.

2. Iron or steel does not replace the copper from thiosulphate solutions in which cuprous chloride has been dissolved.

3. Measurements of the potential of the copper electrode in solutions containing varying concentrations of sodium thiosulphate and cuprous chloride, indicate a far more negative value than would be obtained if no complex ions were formed in the solutions.

For pure copper deposition, a solution containing only sodium thiosulphate and cuprous chloride is satisfactory. The maximum current density is limited by the concentration of the cuprous chloride and the sodium thiosulphate, and by the temperature of the bath. Above a certain value of current density, a deposit of cuprous sulphide forms onthe cathode. The value of the maximum current density is increased by increasing the temperature or the concentration of cuprous chloride. The concentration of the sodium thiosulphate (hypo) should be at least 200 g. per liter of solution, or the solubility of the complex copper salt will be too small to permit a desirable current density. A desirable composition for a copper bath would be:

' NazSzOsfiHzO, (400 g. per liter of solution), C112C12(60 g. per liter of solution). Current densities from 14.5 amp. per sq. dm. could be used at 30 degrees C. The voltage drop across the bath'is less than a volt.

No H2 is set free in this bath, and current efficiencies very close to are obtained.

For alloy deposition, other metal chlorides or sulphates are added to the copper bath. In the deposition of copper-nickelalloys, nickel chloride is used instead of the sulphate in order to insure good anode corrosion, for sulphates have a tendency to cause anode passivity. No H2 is liberated at the current densities which give satisfactory deposits. Consequently, very good current efiicienciesare realized. The addition of considerable quantities of ammonium chloride to the baths is beneficial for deposition at the higher current densities, but quitedetrimental ior deposition at low current densities. Current density does not affect the composition of the deposits to any great extent. The best deposits appear to be obtained at room temperature. The composition of the deposit is best altered by using a different composition of the bath.

In the deposition of copper-zinc alloys, either zinc chloride or sulphate may be used. The presence of ammonium salts or other salts which will maintain the solution slightly acid is very desirable, especially if a, high concentration of zinc salt is used. H2 is liberated innearly all of the baths, but current efliciencies of at least 80% are obtainable even at high current densities. The composition of the deposit may be varied over a wide range by changing the current density or temperature in some of the baths, while in others, the composition remains nearly constant for a large change in current density. An increase in temperature always causes an increase in the copper content of the deposit. The current densities allowable with the baths for the deposition of copper-zinc alloys are higher than with the cyanide baths. For example, a cyanide bath for the deposition of brass will give good deposits at a current density of about 0.3 amp. per sq. dm., while a thiosulphate bath for a similar alloy permits current densities as high as 2 amp. per

sq. dm. With alloys richer in zinc, still higher current densities can be used.

In the deposition of copper and copper alloys, air agitation can be used. It allows a higher current density to be used, and increases the copper content of the deposit if an alloy is being deposited. The use of cuprous chloride, free as possible from cupric compounds, to form the complex copper thiosulphate ion appears to give the best results. A process for the preparation of the cuprous chloride which I employ is disclosed and claimed in my co-pending application, Serial Number 623,295, filed July 18, 1932. Baths prepared with impure cuprous chloride frequently decompose continually with the formation of a brown or black precipitate. -The specific resistivity of these baths ranges from 3 to 6 ohm-cm. The voltage drop across the baths,

even at the highest current densities is only slightly over 1 volt.

It is to be understood that this method of depositingcopper and copper alloys is not to be restricted by any of the following factors:

1. Concentration of the constituents of the bath;

2. Current density;

3. Temperature;

4. Composition of the alloy deposited, excep that it must contain copper.

I claim:

1. An electrolyte for depositing a copper coating,comprising a solution of a soluble thiosulphate and pure cuprous chloride.

2. An electrolyte for depositing a copper coating, comprising a solution of sodium thiosulphate substantially in the proportion of 400 grams per 

